Field of the Invention
The invention relates to a method for acquiring magnetic resonance data of an object to be examined, of the type wherein two-dimensional k-space is scanned along lines extending in a readout direction within a data entry trajectory. The invention further relates to a magnetic resonance apparatus for implementing such a method.
Description of the Prior Art
Magnetic resonance imaging is widely known and has become an established modality in medical examinations. In magnetic resonance imaging, nuclear spins in an object to be examined are aligned by means of a basic magnetic field, the so-called B0 field, and are excited (deflected out of the aforementioned alignment) by radio-frequency pulses, i.e. a field that changes rapidly over time, the so-called B1 field. During the relaxation (return to alignment) of the spins, magnetic resonance signals, which are detected by a radio-frequency coil arrangement, are produced. Data corresponding to the detected magnetic resonance signals are entered into k-space, and are transformed by a Fourier transform into the image domain in order to obtain a magnetic resonance image. In order to fill k-space with the data, it is known to enter the data at different points along a k-space trajectory or recording trajectory that is produced by gradient pulses. This process, i.e. the measurement of a complete recording trajectory in the k-space, requires a certain length of time.
Acceleration of the acquisition of magnetic resonance data is an important aspect of clinical magnetic resonance imaging. For the user, the acquisition speed not only considerably influences patient throughput in the magnetic resonance apparatus, but also defines how long an individual patient has to remain in the magnetic resonance apparatus as the object to be examined. Thus, the saving in terms of throughput and the differentiation in terms of patient comfort are equally relevant.
In this case, proposals have been made as to how magnetic resonance imaging may be accelerated. Parallel imaging is an example whereby different regions of the object to be examined are excited simultaneously and read out. Also, many different proposals have been made that relate to the reconstruction of an image from a k-space data set that is undersampled, i.e., not every possible data entry point in k-space has been filled with a data entry.
A specific form of undersampling of the k-space is known by the term “Compressed Sensing” (CS), see for example the article by Michael Lustig et al., “Sparse MRI: The Application of Compressed Sensing for Rapid MR Imaging”, Magnetic Resonance in Medicine 58:1182-1195 (2007). In this case, an iterative reconstruction is defined by making assumptions about the result space of the imaging, requiring an incomplete, pseudo-randomized scanning of the k-space, wherein the term “Compressed Sensing” in the narrower sense denotes the application of the L1 norm in iterative reconstruction. The practical embodiment of “Compressed Sensing” proposes that individual recording sections, for example lines in the k-space, are randomly omitted, wherein in three-dimensional measurements it is possible to calculate in a simple manner the missing information by iterative reconstruction so that artifact-free images are produced. Two-dimensional measurements are generally carried out along lines in the readout direction in the k-space. The random omission of lines in this case, as it was possible to show, does not lead to artifact-free image results as the undersampling of the k-space does not occur sufficiently randomly. An application of “Compressed Sensing” in spatial two-dimensional imaging is, therefore, only possible in real-time dynamic processes such as angiographs, or recordings of movement processes, as then the time is able to be used as the third dimension.
In summary, therefore, the known methods provide solutions for real-time dynamic processes and for three-dimensional magnetic resonance sequences. However, these solutions, in particular “Compressed Sensing”, are not able to be used for conventional static two-dimensional magnetic resonance imaging which represents the majority of acquisition processes.